This invention relates to aircraft instruments.
Airspeed indicators have a pointer that is moved over a scale calibrated in knots in accordance with the airspeed of the aircraft. Such indicators on high-speed aircraft can also have a separate scale calibrated in Mach number so that this value can readily be ascertained. Mach number, however, is not directly related to airspeed but is also dependent on ambient pressure and hence, altitude. The Mach number scale is therefore usually provided on a moveable dial that is rotated in accordance with the altitude of the aircraft so as to compensate for this effect. Instruments of this kind commonly have a marker that is set against the highest desired airspeed and Mach number at which the aircraft should be flown, so that the pilot can readily determine when he is exceeding or approaching this speed. The maximum speed is dependent upon the ability of the aircraft to withstand the strain and forces exerted on it, and has been found to be more closely related to Mach number than actual airspeed, especially at higher altitudes. Accordingly, Mach airspeed indicators usually have a maximum speed marker that is set against a fixed value of airspeed in knots, below a certain altitude, and is then rotated together with the Mach scale as altitude changes at greater heights. In this way, at high altitudes, the marker moves against the airspeed/knots scale upon change in altitude. Various gearing arrangements to achieve this modification at higher altitudes have been used. While it has been relatively simple to modify the setting of the maximum speed marker against the airspeed scale, from the exterior of the instrument case, it has been considerably more difficult to set the value at which the marker will be taken up by the Mach scale and set against a constant Mach number. In the past this has had to be done by an engineer, by opening the instrument case and appropriately modifying the mechanism. In many cases it is desired to be able readily to change also the maximum Mach number setting for different circumstances, such as, for example, reducing the Mach number when the aircraft is being flown by an inexperienced pilot.